A Look at the AMD Zen 2 Core • Hacker News Board

Re: A Look at the AMD Zen 2 Core

#21

Post by jchw » Sat, Jul 06, 2019, 5:19 PM UTC

garkin wrote: post #18

Zen 2 is very good in number crunching and synthetics. But it has a problem - terrible memory latency. 70ns with 3600cl16. ( https://www.userbenchmark.com/UserRun/18168279 ) It distills to a not-so-good gaming frame times. It's 64mb L3 cache ( https://en.wikichip.org/wiki/amd/ryzen_9/3900x ) helps only partially. Few games will suffer greatly from it, but there are several titles with RAM bottlenecks, like PUBG and F…

Not sure why your original comment disappeared.

I was kind of curious what the latencies might be for other contemporary processors/builds, and I'm not sure 70ns is actually really outside the normal margins.

Here's an i7-8700k build that is already pretty close to 70ns: https://www.userbenchmark.com/UserRun/18173216 - Also acknowledging that, this is not the 'best case' performance. But seems to be not so unusual either.

(edit: Removed section about latency ladder as I just realized it was caching and not system memory latency that we were most likely seeing initially, and therefore not terribly relevant.)

As far as I can tell, Zen 1 has similar latency characteristics[0][1]... so I guess I won't notice any degradation when upgrading to Zen 2.

[0]: https://www.userbenchmark.com/UserRun/18173307

[1]: https://www.userbenchmark.com/UserRun/18173240

Re: A Look at the AMD Zen 2 Core

#22

Post by [deleted] » Sat, Jul 06, 2019, 5:20 PM UTC

garkin wrote: post #18

Zen 2 is very good in number crunching and synthetics. But it has a problem - terrible memory latency. 70ns with 3600cl16. ( https://www.userbenchmark.com/UserRun/18168279 ) It distills to a not-so-good gaming frame times. It's 64mb L3 cache ( https://en.wikichip.org/wiki/amd/ryzen_9/3900x ) helps only partially. Few games will suffer greatly from it, but there are several titles with RAM bottlenecks, like PUBG and F…

[deleted]

Re: A Look at the AMD Zen 2 Core

#23

Post by jchw » Sat, Jul 06, 2019, 5:26 PM UTC

demilicious wrote: post #20

garkin wrote: post #18

Zen 2 is very good in number crunching and synthetics. But it has a problem - terrible memory latency. 70ns with 3600cl16. ( https://www.userbenchmark.com/UserRun/18168279 ) It distills to a not-so-good gaming frame times. It's 64mb L3 cache ( https://en.wikichip.org/wiki/amd/ryzen_9/3900x ) helps only partially. Few games will suffer greatly from it, but there are several titles with RAM bottlenecks, like PUBG and F…

As far as I know, these processors are not yet released. What's the confidence level that this user benchmark will be indicative of real life expected performance? It seems implausible that this user benchmark is a good indicator. The Zen 1 architecture exhibited nothing of the sort[0] -- it would be an order of magnitude performance regression. I expect we'll start to see more accurate tests once the processors are…

Well, to be devil's advocate: just because AMD improved their memory latency does not mean it was state-of-the-art. Indeed, looking at some random user benchmarks of Zen 1, it looks like the memory latency is similar with Zen 1. (See my reply to parent for a couple links, but I think you could also just find random Ryzen 2700X benches and see the same.)

Of course, the effects of ~20ns more latency on system memory accesses may not be as easy to observe in practice, especially if throughput is excellent. But we'll see, I guess; 'gaming' benchmarks should be a good test. (Meanwhile, I'm just going to be happy with Zen 2 if it provides a large speedup in compile times.)

Re: A Look at the AMD Zen 2 Core

#24

Post by demilicious » Sat, Jul 06, 2019, 5:30 PM UTC

jchw wrote: post #23

demilicious wrote: post #20

Earlier quoted context omitted.

As far as I know, these processors are not yet released. What's the confidence level that this user benchmark will be indicative of real life expected performance? It seems implausible that this user benchmark is a good indicator. The Zen 1 architecture exhibited nothing of the sort[0] -- it would be an order of magnitude performance regression. I expect we'll start to see more accurate tests once the processors are…

Well, to be devil's advocate: just because AMD improved their memory latency does not mean it was state-of-the-art. Indeed, looking at some random user benchmarks of Zen 1, it looks like the memory latency is similar with Zen 1. (See my reply to parent for a couple links, but I think you could also just find random Ryzen 2700X benches and see the same.) Of course, the effects of ~20ns more latency on system memory ac…

That makes sense. I was mostly looking at one of the Tom's Hardware benchmarks in particular; I don't know what the userbenchmark is actually _doing_. So it's probably more fair to compare userbenchmark to userbenchmark like you did.

However, I think it's still true that the actual release of the Zen 2 processors will make userbenchmark scores much more reliable as the scores regress to the mean.

Re: A Look at the AMD Zen 2 Core

#25

Post by fgonzag » Sat, Jul 06, 2019, 5:35 PM UTC

ademup wrote: post #4

"Zen employs a dynamic predictor known as a hashed perceptron." When will Hollywood tap into this wealth of cool vocabulary? Is this a gimmicky marketing term, or is it logical\descriptive\rational?

A perceptron is a simple type of neural network that is very fast to use when already trained. So it is a real network, I don't know what hashed perceptron is but I'd guess it would have to do something with hashing the inputs to the perceptron?

Re: A Look at the AMD Zen 2 Core

#26

Post by josalhor » Sat, Jul 06, 2019, 5:42 PM UTC

garkin wrote: post #18

Zen 2 is very good in number crunching and synthetics. But it has a problem - terrible memory latency. 70ns with 3600cl16. ( https://www.userbenchmark.com/UserRun/18168279 ) It distills to a not-so-good gaming frame times. It's 64mb L3 cache ( https://en.wikichip.org/wiki/amd/ryzen_9/3900x ) helps only partially. Few games will suffer greatly from it, but there are several titles with RAM bottlenecks, like PUBG and F…

As other users have pointed out with concrete sources, this comment seems to come to the conclusion that Zen 2 will not be competitive enough based on cache information. But looks to me that the timings are much lower on 16 MiB, and that's on a dedicated benchmark.

It also seems to ignore the comment on userbenchmark itself that indicates that latency is less of a bottleneck than bandwidth.

I'll say coming to the conclusion that Zen 2 is less than competitive in comparison to Intel offering a day ahead of launch is more than brave.

Re: A Look at the AMD Zen 2 Core

#27

Post by tempguy9999 » Sat, Jul 06, 2019, 5:46 PM UTC

atq2119 wrote: post #19

tempguy9999 wrote: post #15

Earlier quoted context omitted.

The parent's ROB is the reorder buffer. AIUI it causes the instructions to be retired in order (with exceptions stored until retirement, then exposed). The original question, though is how a particular u-op is mapped back to the original macro-instruction, so we know what macro-instruction excepted. And I don't know. I guess is if each u-op is tagged with the instruction address within the process, that would do, but…

Perhaps there are two copies of the program pointer, one at the "top of the pipe" updated by instruction decode and branch prediction, and one at the "bottom of the pipe" updated by the ROB. Then uops only need to carry the amount by which the program counter is advanced, and certain events can cause a pipeline flush and copy the bottom of pipe version of the counter to the top of the pipe. But that's also all specul…

> But that's also all speculation :)

How appropriate!

But, your idea I think will not work in general where there's branching.

Re: A Look at the AMD Zen 2 Core

#28

Post by Zardoz84 » Sat, Jul 06, 2019, 5:47 PM UTC

garkin wrote: post #18

Zen 2 is very good in number crunching and synthetics. But it has a problem - terrible memory latency. 70ns with 3600cl16. ( https://www.userbenchmark.com/UserRun/18168279 ) It distills to a not-so-good gaming frame times. It's 64mb L3 cache ( https://en.wikichip.org/wiki/amd/ryzen_9/3900x ) helps only partially. Few games will suffer greatly from it, but there are several titles with RAM bottlenecks, like PUBG and F…

if a old and outdated FX8370 can handle modern games, I think that a Zen 2 would not have any problem.

Re: A Look at the AMD Zen 2 Core

#29

Post by mtgx » Sat, Jul 06, 2019, 5:52 PM UTC

ademup wrote: post #4

"Zen employs a dynamic predictor known as a hashed perceptron." When will Hollywood tap into this wealth of cool vocabulary? Is this a gimmicky marketing term, or is it logical\descriptive\rational?

Hollywood might go for "GameCache" instead.

Re: A Look at the AMD Zen 2 Core

#30

Post by mtgx » Sat, Jul 06, 2019, 5:53 PM UTC

I wish AMD enabled Secure Encrypted Memory by default for all devices, and also bring Secure Encrypted Virtualization (or something similar) to consumer Ryzen chips, too.

Even Microsoft has made VM-sandboxing a consumer feature. It's time for AMD to join the bandwagon and help along the way, not keep the virtualization/other security features exclusive to server customers.